1. Field of the Invention
The present invention relates to virtual image systems.
2. Background Art
It is difficult to display three dimensional images on most present display devices. Images may be displayed on a two dimensional display device such as a CRT, and the images may be displayed in perspective so as to give the appearance of three dimensions. However, the image size must be adjusted to provide perspective. Portions of an image intended to appear more distant must be reduced in size relative to portions intended to appear closer. While two dimensional images in perspective may have the correct proportions to simulate three dimensional images, they do not provide parallax. Without the proper parallax, a viewer can easily distinguish a two dimensional image in perspective from a true three dimensional image.
Other approaches have been used to provide parallax as well as proportion. Mirror mazes or labyrinths have been constructed to provide the illusion of hallways, rooms and other objects in locations where they did not actually exist. An article entitled "The Amateur Scientist; Mirrors Make a Maze So Bewildering That the Explorer Must Rely on a Map" by Jearl Walker on pages 120-126 of Scientific American, volume 254 (June 1986) discloses a number of mirror labyrinths and method for designing and analyzing mirror labyrinths. The mirror labyrinths are constructed using mirrors aligned along the edges of equilateral triangles. Thus, the mirrors meet each other at angles that are multiples of 60.degree.. The mirrors produce regularly repeated images around the borders of a "hallway" when viewed from the proper location and direction. The image that appears at the distant end of a "hallway" can be controlled. However, traditional mirror labyrinths do not provide for images having some degree of transparency to be interposed between the viewer and the distant end of a "hallway" such that objects at the distant end of the hallway can still be seen through the interposed images. Furthermore, traditional mirror labyrinths do not allow images to pass through mirrors and do not provide images originating behind exterior mirrors to be seen within the labyrinth.
Other approaches have been used to simulate stereo vision, where each eye sees a different image so as to provide a three dimensional effect. One prior art method of providing stereo vision involved a viewer with spectrally filtered eyeglasses viewing an image having spectrally encoded stereo information. For this approach, the viewer wears eyeglasses that typically have a red lens over one eye and a green or blue lens over the opposite eye. The viewer views an image that includes a left component and a right component. The left component is of the color of the lens covering the left eye, while the right component is of the color of the lens covering the right eye. The left component provides the left eye with a view representative of what the left eye would see if the image were three dimensional. The right component provides the right eye with a view representative of what the right eye would see if the image were three dimensional. The views seen by the right and left eyes are mentally combined to form a three dimensional perception of the image. However, since colors are used to encode stereo vision information, the colors of the image must be carefully controlled, and the image cannot be naturally colored. Thus, while the images may appear three dimensional, they are unnaturally colored. Furthermore, the three dimensional effect is lost unless the viewer wears the filtering eyeglasses.
Another approach involves the viewer wearing orthogonally oriented polarizing filters over each eye. An image having stereo vision information encoded with orthogonal polarization is viewed by the viewer. Stereo vision information relevant to the left eye is encoded with a polarization matching the polarization of the lens over the left eye, while stereo vision information relevant to the right eye is encoded with a polarization matching the polarization of the lens over the right eye. Thus, the left and right eyes receive their respective stereo vision information, which is mentally combined to form a three dimensional perception. However, the three dimensional effect is lost unless the viewer wears cross polarized eyeglasses.
Another approach involves a viewer wearing (liquid crystal display) LCD shuttered eyeglasses and viewing an image that alternates between a left component and a fight component while the LCD shutters alternate between blocking vision of the right eye and left eye, respectively. The eyeglasses contain individually controllable LCD shutters over the left eye and over the right eye. The eyeglasses are used to view a display device, such as a cathode ray tube (CRT) that can rapidly change the image it is displaying. When the display device displays a left component of stereo vision information, the LCD shutter over the left eye is opened and the LCD shutter over the right eye is closed. When the display device displays a right component of stereo vision information, the LCD shutter over the right eye is opened and the LCD shutter over the left eye is closed. Thus, the left and right eyes receive their respective left and right components of stereo vision information. The left and right components of stereo vision information are combined mentally to form a three dimensional perception. However, the three dimensional effect is lost unless the viewer wears the LCD shutter eyeglasses.
One method of the prior art for causing a semi-transparent image to appear in front of a background is known as "Pepper's Ghost" and involves reflecting an image from a beam splitter placed in front of a scene. "Pepper's Ghost" provides only a single reflection. It does not allow multiple planes of transparent images. Furthermore, it cannot produce images that pass through each other in a direction parallel to the optical path to the viewer. Also, to produce expansive three dimensional images, the "Pepper's Ghost" technique requires a large three dimensional volume.
Another prior art method has been used for displaying the image of an object located behind a beam splitter. This method involves placing an object in an opaque enclosure having one side constructed of a beam splitter. A lamp that may be dimmed is placed inside the enclosure to illuminate the surface of the object facing the beam splitter. When the lamp is off, the object is not visible from outside the enclosure, and the beam splitter appears to be a mirror when viewed from the outside. When the lamp is on, the object can be seen through the beam splitter. This method does not allow multiple images to be superimposed on one another. This method cannot provide multiple planes of transparent images. Also, this method does not allow the image to be made to disappear without reverting to a mirror condition. Additionally, this method cannot produce images that pass through each other in a direction parallel to the optical path to the viewer. Furthermore, to provide large expansive three dimensional images, this technique requires a large three dimensional volume.